1. Field of the Invention
The present invention relates to a display element for displaying and recording an image, a document, and code information such as bar code and performing an optical information processing such as light modulation and optical switching using an optical change such as light transmission, light reflection, and light refraction, and a manufacturing method therefor.
2. Description of the Related Art
Conventionally, various display elements using a liquid display material such as a liquid crystal display element and an electrophoresis display element have been employed. To put such a display element into practical use, it is required to have a process of enclosing a display material into a “container” like a pair of substrates with the periphery sealed. Therefore, a process of forming the container, and a process of enclosing the display material into the container are required. For example, it takes several to several tens hours to make a filling process of the liquid crystal material in the liquid crystal display element, leading to a main cause of impeding the productivity.
In these display elements, it has been required to place a spacer member in a display layer to make the thickness of the display layer constant. Also, it has been required to form a flow preventing partition for preventing the flow of the display material owing to an external pressure or the maldistribution of the display material caused thereby. Therefore, there has been a problem that number of members and number of manufacturing processes are increased to raise the cost.
On one hand, a display element of binder dispersion type is well known in which a display material is dispersed into the binder and held. As one example, micro-capsules carrying the display material are dispersed into the binder material such as resin. For example, there are an electrophoresis display element in which an insulating oil containing colored charged particles is enclosed into micro-capsules, a magnetophoresis liquid crystal element in which an insulating oil containing magnetic particles is enclosed into micro-capsules, and a liquid crystal element in which a liquid crystal material is enclosed into micro-capsules. As another example, a display material treated in a liquid is dispersed into a solution of binder material incompatible with the display material and applied and dried. For example, there is a liquid crystal element in which a liquid crystal material is dispersed into an aqueous solution of water soluble resin, applied and dried. As a further example, a binder material and a display material are dissolved uniformly, using solving means such as a solvent or heating, and then an external stimulus such as heat or light is applied to cause the phase separation of the display material and the binder. For example, there are well known a liquid crystal display element of polymer dispersed type in which a resin and a liquid crystal material are phase separated from a cosolvent solution by solvent drying, and a liquid crystal display element of polymer dispersed type that is produced by separating a photochemical polymerization phase from a mixed solution of a photochemical polymerization monomer and the liquid material.
The above display element of binder dispersion type is obtained in the form in which the display material is dispersed into and enclosed in the binder solution. Therefore, there is no need for a process of forming the container and a process of enclosing the display material into the container. Also, since the thickness is kept owing to the strength of binder, there is no need for disposing a spacer. Furthermore, since the binder acts as a partition wall, there is no need for forming the flow preventing wall. Therefore, the construction is simplified, the number of manufacturing processes is reduced, and the cost is lowered.
In this display element of binder dispersion type, it is common to form a display layer by applying a binder solution having the display material dispersed on a substrate. To protect this applied face, another substrate may be bonded on the display layer. Particularly, when the display material is a material of voltage response type and it is desired to display in a desired or arbitrary pattern, the display layer may be held between a pair of substrates having a transparent conductive film such as ITO (Indium Tin Oxide) formed as an electrode to apply a voltage to the display layer. In this case, first of all, the display layer is applied to and formed on one substrate with electrode, and then the other substrate with electrode is bonded thereon to produce the display element.
FIG. 9 is a cross-sectional view showing one example of a display element according to the related art. In FIG. 9, reference numeral 1 denotes a display element, 2 denotes a voltage applying portion, 3 denotes an exposure portion, 4 denotes a control portion, 11a, 11b denote substrates, 12a, 12b denote electrodes, 13 denotes a display layer, 14 denotes a light shielding layer, and 15 denotes a photoconductive layer. In FIG. 9, as one example of the display element according to the related art, a display element of optically writing type using a cholesteric liquid crystal is shown. The display element with this construction is disclosed in JP-A-2002-6342.
The display element 1 has a plurality of display function layers carried by the pair of substrates 11a and 11b to implement the display function. In this example, the display function layers include the electrode 12a, the display layer 13, the light shielding layer 14, the photoconductive layer 15 and the electrode 12b, which are provided in this order. The light shielding layer 14 is provided to shield a transmitted light from the back side thereof and prevent a display contrast ratio from being lowered, in a display element of reflection type for displaying with the reflection of external light or a display element of spontaneous emission type. Also, in this example, the photoconductive layer 15 is provided as a switching layer for switching a display mode, as will be described later.
The voltage applying portion 2 is connected between the electrodes 12a and 12b. The exposure portion 3 for illuminating a light image of an image or characters is disposed facing the photoconductive layer 15. The control portion 4 controls the voltage applying portion 2 and the exposure portion 3 so that a voltage is applied in synchronization with exposure.
This display element 1 operates as follows. The voltage applying portion 2 applies a voltage between the electrodes 12a and 12b, while the exposure portion 3 applies a light image to the photoconductive layer 15. Then, a voltage divided by the display layer 13, the light shielding layer 14 and the photoconductive layer 15 is applied to the display layer 13 having a liquid crystal. At this time, an electrical resistance value of the photoconductive layer 15 varies at different positions, depending on the light intensity of the light image. That is, the resistance is low at a position where the applied light intensity is strong, while the resistance is high at a position where the light intensity is weak. In response thereto, the divided voltage on the display layer 13 is high at the position where the light intensity is large, while the divided voltage on the display layer 13 is small at the position where the light intensity is weak, thereby causing a change in the orientation of liquid crystal, which appears as a change in the reflection factor. The cholesteric liquid crystal has a memory property of retaining the display even after removal of the voltage. Therefore, this display element acts as a display and recording element.
FIG. 10 is an explanatory diagram for explaining one example of the manufacturing process for the display element according to the related art. The display element 1 as shown in FIG. 9 is produced in accordance with the following procedure. First of all, the electrode 12a is formed on the substrate 11a, as shown in FIG. 10(A). Then, a dispersion liquid containing micro-capsules including cholesteric liquid crystal as the core material that are dispersed into a binder resin solution is applied to and dried on the substrate 11a with the electrode 12a formed thereon, to form the display layer 13, as shown in FIG. 10(B).
On the other hand, the electrode 12b is formed on the substrate 11b, as shown in FIG. 10(C). An organic photoconductive material solution is applied thereon to form the photoconductive layer 15, as shown in FIG. 10(D). Subsequently, a dispersion liquid having a black coloring matter dispersed into the binder resin solution is applied to form the light shielding layer 14, as shown in FIG. 10(E). An adhesive, not shown, is coated around the periphery of the substrate to bond the substrate 1a of FIG. 10(B) and the substrate 1b of FIG. 10(E) to produce the display element 1 as shown in FIG. 10(F).
In this manufacturing process, particles of the dispersed display material is reflected such that the display layer 13 of binder dispersion type as shown in FIG. 10(B) has roughness on the surface. Therefore, air bubbles caused by the roughness on the surface may remain when bonding the substrate 11a and the substrate 11b. The remaining bubbles reflect light to lower the display contrast ratio and/or to increase the image noise, resulting in a problem that the display quality is degraded.
The roughness on the surface of the display layer can be relieved to some extent by increasing a mixture ratio of binder to the display material and/or reducing the particle diameter of dispersed display material. However, if the mixture ratio of binder is increased, the voltage (driving voltage) required for the display is caused to increase. Also, even if the particle diameter of display material is smaller, there is a limitation in relieving the roughness. Because it is necessary to have the particle diameter of at least several μm or greater to obtain an excellent display contract ratio using the cholesteric liquid crystal. Furthermore, a flattening layer for filling up the roughness on the display layer 13 may be possibly formed. For example, adhesives may fill up the roughness. However, considerable film thickness is required to provide the flattening layer. Although there is an effect for relieving the roughness, there is a problem of increasing the driving voltage as with the case where the mixture ratio of binder is increased.